1. Field of the Invention
This invention relates to an apparatus for the parallel operation of a plurality of triport uninterruptable power source devices arranged in parallel connection and adapted to feed power to a load.
2. Description of the Prior Art
The widespread instance of computers and microprocessors has been of increasing importance to uninterruptable power source devices as operating power sources for such electronic devices. When the power required by a given load is so large as to surpass the capacity of one uninterruptable power source device, it becomes necessary to provide an increased magnitude of power by parallel operation of a plurality of power source devices.
In most cases, such an uninterruptable power source device for parallel operation as mentioned above is generally configured as a conventional uninterruptable power source comprising a battery driven inverter device and a commercial power source which are connected in parallel with each other, and some additional battery-driven inverter devices connected in parallel to that conventional uninterruptable power source (as reported in the treatise titled "On the Paralleling of UPS systems" by Clement Fontaine at the 1986 International Telecommunications Energy Conference, for example).
The use of such uninterruptable power source devices (hereinafter referred to briefly as "UPS") found in the prior art as described above entails the following problems.
Firstly, the burdens of load laid on the individual UPS's are desired to be proportionate to the inverter capacities of the USP's. The control necessary for proportionate load distribution is not easy to effect. In conventional UPS's, the output currents depend on the phases of inverter drive pulses and increase in proportion to the phases advance. For the purpose of proportional load distribution mentioned above, it is necessary to detect the output currents of the inverters in parallel operation and control the phases of the inverter drive pulses in accordance with the inverter capacities and the detected output currents. The circuit for this control and the method for its operation are very intricate. Thus, the control cannot be easily carried out with high reliability.
Secondly, the circulating currents which occur between the inverters of the UPS's under parallel operation when phase differences occur between the output voltages of the inverters are suppressed only with difficulty. The suppression of the circulating current may be possible to be attained by inserting a current limiting element such as a reactor in each of the inverter circuits or by detecting the circulating current and accordingly adjusting the drive timing (phase) of the inverter. In these methods, there is a problem in that the number of circuit elements required for a control is increased or the problem in that the circuit for the timing adjustment is complicated.
Thirdly, concerning the synchronous operation required for the inverters of the UPS's under parallel operation, while the conventional UPS's are controlled with high responsiveness because of their small internal inductance, they have the problem that a disruption of the synchronization among the plurality of inverters results in a very large circulating current. To cope with this problem, the synchronization among the inverters must be controlled very accurately and quickly. The circuit for realizing such accurate and quick control of the synchronization as mentioned above necessitates advanced design and complicated configuration and suffers from high cost and poor reliability.
Fourthly, when any of the UPS's under parallel operation develop trouble, it must be removed from such operation parallel as soon as possible. Since the conventional UPS has a small internal inductance as mentioned above, an accidental short circuit may possibly result in an excessive rate of increase in the short-circuit current. The preclusion of such trouble calls for early detection of the undue short-circuit current and quick removal of the UPS in trouble. The circuitry capable of fulfilling this requirement is so complicated as to entail the disadvantage of high cost and poor reliability.